Implement file transfer feature

This will add separate "tfer" buffer with all necessary controls for
managing files and /send command to start transfer, which you can use
either from profile buffer or from private messages buffer; in case of
incoming files the buffer will be created as well if doesn't exist yet.
Every callbacks were defined according to the specification and this
implementation is fully corresponding to the documented behavior including
streams.

src/twc-utils.c

@@ -232,3 +232,82 @@ twc_set_buffer_logging(struct t_gui_buffer *buffer, bool logging)
     return weechat_hook_signal_send(signal, WEECHAT_HOOK_SIGNAL_POINTER,
                                     buffer);
 }
+
+/**
+ * These following twc_tox_err_file_* functions convert enum TOX_ERR_FILE_*
+ * error codes to meaningful messages of type char *.
+ */
+
+char *
+twc_tox_err_file_control(enum TOX_ERR_FILE_CONTROL error)
+{
+    char *messages[] = {
+        "success",
+        "the friend number passed did not designate a valid friend",
+        "this client is currently not connected to the friend",
+        "no file transfer with the given file number was found for the given friend",
+        "a RESUME control was sent, but the file transfer is running normally",
+        "A RESUME control was sent, but the file transfer was paused by the other party",
+        "a PAUSE control was sent, but the file transfer was already paused",
+        "packet queue is full"
+    };
+    return messages[error];
+}
+
+char *
+twc_tox_err_file_get(enum TOX_ERR_FILE_GET error)
+{
+    char *messages[] = {
+        "success",
+        "one of the arguments to the function was NULL when it was not expected",
+        "the friend number passed did not designate a valid friend",
+        "no file transfer with the given number was found for the given friend"
+    };
+    return messages[error];
+}
+
+char *
+twc_tox_err_file_seek(enum TOX_ERR_FILE_SEEK error)
+{
+    char *messages[] = {
+        "success",
+        "the friend number passed did not designate a valid friend",
+        "the client is currently not connected to the friend",
+        "no file transfer with the given file number was found for the given friend",
+        "file was not in a state where it could be seeked",
+        "seek position was invalid",
+        "packet queue is full"
+    };
+    return messages[error];
+}
+
+char *
+twc_tox_err_file_send(enum TOX_ERR_FILE_SEND error)
+{
+    char *messages[] = {
+        "success",
+        "one of the arguments of the function was NULL when it was not expected",
+        "the friend number passed did not designate a valid friend",
+        "this client is currently not connected to the friend",
+        "filename lenth exceeded TOX_MAX_FILENAME_LENGTH bytes",
+        "too many ongoing transfers"
+    };
+    return messages[error];
+}
+
+char *
+twc_tox_err_file_send_chunk(enum TOX_ERR_FILE_SEND_CHUNK error)
+{
+    char *messages[] = {
+        "success",
+        "the length parameter was non-zero, but data was NULL",
+        "the friend number passed did not designate a valid friend",
+        "this client is currently not connected to the friend",
+        "no file transfer with the given file number was found for the given friend",
+        "not called from the request chunk callback",
+        "attempted to send more or less data than requested",
+        "packet queue is full",
+        "position parameter was wrong"
+    };
+    return messages[error];
+}